An addon that reminds you that your Flask has expired is very handy. And there are quite a few on WoWInterface, but I have never found them "pretty". What I wanted was an addon that reminds you in a subtle yet noticeable way that you haven't got a Flask up. Then I stumbled upon the addon of sigmalmtd: DivinePleaReminder: An addon that flashes the button of Divine Plea when it isn't active.

I threw the two together and voilą: An addon that Flashes your Flask button on your action bar if you are in a raid and your Flask is not active.

This AddOn is lightweight and stateless, meaning it has low memory and CPU usage, does not have configuration slash-commands, and does not store any SavedVariables.

Note: For Corn's Flask Reminder to work you need to have your Flask on your Action Bar. It will work if you have multiple instances of the spell, and if you move the spell around. However, it will not be able to find macros that in some way cast your flask, and thus will not highlight those buttons.